Transparent wood composite, systems and method of fabrication

ABSTRACT

Highly transparent (up to 92% light transmittance) wood composites have been developed. The process of fabricating the transparent wood composites includes lignin removal followed by index-matching polymer infiltration resulted in fabrication of the transparent wood composites with preserved naturally aligned nanoscale fibers. The thickness of the transparent wood composite can be tailored by controlling the thickness of the initial wood substrate. The optical transmittance can be tailored by selecting infiltrating polymers with different refractive indices. The transparent wood composites have a range of applications in biodegradable electronics, optoelectronics, as well as structural and energy efficient building materials. By coating the transparent wood composite layer on the surface of GaAs thin film solar cell, an 18% enhancement in the overall energy conversion efficiency has been attained.

REFERENCE TO RELATED APPLICATION(S)

This Utility Patent Application is based on the Provisional PatentApplication No. 62/291,151 filed 4 Feb. 2016.

STATEMENT REGARDING FEDERAL SPONSORED RESEARCH OR DEVELOPMENT

This invention was made with government support under FA95501310143awarded by AFOSR. The government has certain rights in the invention.

FIELD OF THE INVENTION

The present invention is directed to light transmitting systems, and inparticular to wood-based light transmitting systems.

Even more in particular, the present invention is directed toanisotropic transparent wood mesoporous composites having unique opticalproperties in a broad wavelength range between 400 nm and 1100 nm whichcan be utilizable for a wide range of optoelectronic and photonicsystems, where light management is crucial for enhanced operationefficiency. The systems provide for high mechanical strength andductility and may be used as energy efficient building materials forguided sunlight transmittance and effective thermal insulation.

The present invention is also directed to a method of fabrication oftransparent wood composites in a two stage process, including extractionof lignin from low tortuosity channels of the natural wood (in the firststage) followed by the second stage for infiltrating of lignin-devoidwood blocks with material(s) having refractive index substantiallymatching the refractive index of the channel walls' cellulose-containingmaterial.

In addition, the present invention is directed to optoelectronic systemsusing a broad range light management layer formed with anisotropictransparent wood composite(s) fabricated in a cost-efficient manner.

The present invention is further directed to energy efficient buildingmaterials for guided sunlight transmittance and effective thermalinsulation using transparent wood composites which, when installed as areplacement of windows and/or rooftops, efficiently harness light forconsistent and uniform indoor lighting.

BACKGROUND OF THE INVENTION

Wood is a widely used structural material that has excellent mechanicalproperties due to the unique structures developed during its naturalgrowth. Depending on their types and geographical differences, differentwoods display a wide variety of mesostructures. For example, soft woodstypically have a more porous structure due to their fast growth. Hardwoods normally have a more dense structure and a higher density comparedto soft wood. Although the large-scale structures in different woods canbe dramatically different, the mesoporous structures of wood sharesimilarities in their hierarchical structures.

An outstanding feature of woods is their structural anisotropy due tothe existence of numerous aligned natural internal channels. Verticallyaligned internal channels in the trunk of trees are used to pump ions,water and other ingredients through the wood trunk to meet the need oftrees for photosynthesis.

Typical wood is mainly composed of cellulose and hemicellulose fibers,and lignin. Wood cell nano- and micro-fibers are naturally aligned alongthe growth direction and form walls of the internal channels. Thefibers' dimensions are typically from 3 mm to 5 mm in length and fromless than 10 μm to 50 μm in diameter. The alignment of cellulosenanofibers together with the strong interactions among the biopolymersin wood is enhanced by lignin which acts as a matrix adhesive, forming atypical fiber reinforced, anisotropic mesostructure. Each wood cellfiber contains multiple microfibers and each microfiber can further bebroken down into nanosized fibers. Wood is either directly used as astructural material or as the rich source from which cellulosemicrofibers are extracted to make non-transparent paper, which is usedwidely in everyday life.

Recently, researchers have begun to look into emerging applications ofbiopolymers from wood, especially cellulose nanofibers (CNF) andcellulose nanocrystals (CNC). CNF and CNC are extremely attractive for abroad range of new applications including green electronics, energystorage and biological devices. Simultaneously, lignin, which is abyproduct of the pulping process, has been explored as a potential lowcost material for making high-performance carbon or energy storageelectrodes.

Natural wood is not transparent for mainly two reasons. First, naturalwood has microsized channels that scatter light in the visible range.Second, lignin infiltrating wood (up to 30% by mass) absorbs visiblelight and leads to the opaque appearance of most woods.

Two major components in wood, cellulose and hemicellulose, areinsulating polymers with extremely low light absorption. Their opticaltransmittance can be tailored to be as clear as plastic and glass, or behazy for different applications. However, extracting CNF from wood is anenergy and time consuming process.

Light management is critical for improving the optoelectronic devices.For example, a range of light trapping strategies have been developed,such as nano-cone structures, nano-dome arrays, nano-tube lattices,nanowires, as well as metallic nanoparticles, which increase the lighttransport path in the active layers to effectively increase energyconversion efficiencies in devices. Bio-inspired approaches have alsobeen used to design advanced nanostructures for light trapping.

Transparent optical material is one of the most important buildingblocks for solar-based energy conversion devices, where glass has longbeen the traditional material. To enable flexibility of optical systems,plastic has been explored with success as a glass replacement foroptical device integrations. However, plastic substrates have intrinsicproblems such as poor thermal stability, difficulty in beingfunctionalized, and adding waste to landfills. Mesoporous wood fibers innature directed to photonics is desirable due to its abundance, uniquehierarchical structure, rich surface chemistry and use of well-developedprocessing of wood. Built by nature, wood has unique mesostructures thatcan lead to advantageous properties such as excellent mechanicalstrength and efficient transport of water and ions.

As promoted by the U.S. Department of Energy (DOE), energy consumptionof buildings is to be reduced by 20% by 2020, and 50% as the long-termgoal. Energy used for lighting and thermal comfort contributes to morethan 50% of the total energy consumption in residential and commercialbuildings. Consequently, conserving air conditioning and lighting usageespecially during daytime can yield substantial savings. Sunlight is thebest, most natural light for most daily living needs. Windows play a keyrole in energy management within buildings. Glass is the most commonlyused material for windows for sunlight harvesting. However, glasswindows suffer from the following problems:

(1) Glass often creates shadowing effects and discomforting glare. Tocreate efficient, uniform, and consistent indoor lighting inside thebuilding, the light harvesting window needs to yield effectivedirectional scattering including a high transparency over visible rangeand a large scattering effect in the forward direction. Currentstrategies used to realize directional scattering often involve complexnanostructures based on Mie scattering or other resonant scatteringeffects where the size of the nanostructures must be finely tuned.Consequently, such techniques show limited capability for large-scalecommercial applications.

(2) Due to the intrinsic high thermal conductivity of glass, one-thirdof the energy used to heat or cool the building is lost throughinefficient glass windows.

(3) Glass is highly brittle and shatters upon sudden impact, which canlead to safety issues.

In contrast to glass, wood is a natural thermal insulator with excellentmechanical strength, which has been used as a structural material forhouses and cabins for thousands of years. It is the hierarchicalstructure and the strong interactions among cellulose, hemicellulose andlignin that leads to excellent mechanical properties in wood. However,natural wood is not transparent due to light absorbing lignin andmicrosized scattering cell lumens.

If wood could be fabricated in a manner to make it transparent, it wouldfind its usefulness in a wide range of applications from everyday uses(such as, for example, wood furniture) to more advanced applications(such as structural materials in automobiles, as building materials, andin optoelectronics, etc.).

It would be highly desirable to fabricate, in a cost-efficient manner,wood based light management materials as an attractive platform foroptoelectronic devices with highly efficient broadband light managementto enhance the light trapping inside active layer in energy conversiondevices (such as, for example, thin film solar cells and photoelectricchemical cells), and to fabricate transparent and translucent woodcomposites which would be useful as energy efficient building materialsfor daylight harvesting and thermal insulation.

SUMMARY OF THE INVENTION

It is therefore an object of the present invention to provide atransparent wood composite material which is fabricated in a costeffective manner and displays extra-ordinary anisotropical optical andmechanical properties which can be used as wood-based light managementmaterial in optoelectronics, for numerous energy conversion devices(such as, for example, thin film solar cells and photoelectric chemicalcells), as well as for use as a building material to efficiently harvestsunlight to provide consistent and uniform indoor lighting.

Transparent wood composites have been fabricated for the first time bythe novel two-stage fabrication process by removing lignin from a woodblock pre-cut from natural block, followed by infiltrating thelignin-devoid wood block with index-matching polymer(s) to achieve highoptical transparency of the wood block. Depending on the direction ofthe wood block cut, different types of transparent wood compositematerial can be fabricated where the natural internal channels in woodalign either perpendicularly to the wood cut plane, or along the woodcut plane, or in other angled relationships therebetween.

The structure-process-properties relationship has been studied in twotypes of the subject wood composites, and it was found that thefabrication processes for the cross-cut and longitudinally cut woodsamples require different fabrication regimes due to the distinctkinetics of the lignin removal and polymer infiltration along the openinternal channels in the cross-cut and longitudinally-cut samples. Theresulting wood composites maintain the original alignment structure ofcellulose inside the cell (channels) walls and display extraordinaryanisotropic optical and mechanical properties.

In one aspect, the present invention is directed to a transparent woodcomposite which comprises:

a wood block of predetermined dimensions pre-cut from a natural wood ina predetermined angular relationship to a direction of natural internalchannels of the natural wood and treated to remove therefrom the naturalwood's lignin, and

a filling polymer having refractive index substantially matching therefractive index of the cellulose-containing material of the internalchannels' walls, and substantially completely infiltrating the internalchannels of the lignin-devoid wood block.

The pre-cut wood block is configured with an upper and a bottom cutplanes spaced apart one from another a pre-determined distance of 100 μmor larger, for example, ranging approximately between 100 μm and 1.4 cmfor different applications. At least one of the upper and bottom cutplanes extends in crossing relationship with the natural internalchannels or substantially therealong, or in other angular relationshiptherebetween.

The filling polymer has refractive index close to the refractive indexof cellulose, i.e., 1.48, and specifically, is chosen to have arefractive index of approximately 1.53 at the light wavelength of λ=550nm.

The filling polymer may be selected from a large group of materialsincluding:

Thermosetting polymers, such as, for example, Polyester fiberglass,Polyurethanes polymers, Vulcanized rubber, Bakelite, Duroplast,Urea-formaldehyde, Melamine resin, Diallyl-phthalate (DAP), Polyimidesand Bismaleimides, Cyanate esters or polycyanurates, Furan resins,Polyester resins, Silicone resins, Benzoxazine resins, Bis-Maleimides(BMI), Cyanate ester resins, Epoxy (Epoxide) resins, Phenolic (PF)resins, Polyester resins, Polyimides, Polyurethane (PUR) resins,Silicone Resins, Vinyl ester resins,

Thermoplastic polymers, such as, for example, Acrylic, ABS, Nylon, PLA,Polybenzimidazole, Polycarbonate, Polyether sulfone, Polyetheretherketone, Polyetherimide, Polyethylene, Polyphenylene oxide, Polyphenylenesulfide, Polypropylene, Polystyrene, Polyvinyl chloride, Teflon,

Cellulose derivatives, such as, for example, Cellulose acetate,Cellulose acetate butyrate, Cellulose triacetate, Methyl cellulose,Hydroxypropyl methyl cellulose, Ethyl cellulose, Hydroxyethyl cellulose,Carboxymethyl cellulose, Dissolved cellulose, Nanofibrillated cellulose,Cellulose nanocrystals,

functional index matching materials, such as, for example, liquidcrystal, pressure/temperature sensing materials, piezoelectricmaterials,

as well as colorless polymer nano-glue, transparent liquid epoxy resinprecursor with low viscosity, a mixture of a resin with non-blushingcycloaliphatic hardener, polyvinylpyrrolidone (PVP), Poly(methylmethacrylate) (PMMA), Poly(vinyl alcohol) (PVA), Polydimethylsiloxane(PDMS), etc.

In another aspect, the present invention is directed to a wood-basedlight transmitting system, which comprises at least one transparent woodcomposite member formed from at least one wood block pre-cut from anatural wood in a predetermined angular relationship to a direction ofnatural internal channels in the natural wood and treated to removelignin therefrom, thus forming lignin-devoid wood block, and

a filling polymer having refractive index substantially matching therefractive index of the cellulose-containing material of the naturalinternal channels' walls and substantially fully infiltrating thenatural internal channels in the lignin-devoid wood block.

The transparent wood composite member has an upper and a bottom planes.The predetermined angular relationship constitutes an angle, forexample, of approximately 90° between the direction of the internalchannels and at least one of the upper and bottom planes of thetransparent wood composite member. Alternatively, the upper and/orbottom planes are cut in a direction substantially coinciding with thedirection of the natural internal channels in the transparent woodcomposite member. Other angular relationship between 0° and 90° betweenthe wood block's planes and the direction of the interval channels alsois contemplated in the subject system and method.

The transparent wood composite member may be configured as a thick orthin block or a layer having a length of 1 mm or larger, a width of 1 mmor larger, and a thickness of 100 μm or larger.

The resulting transparent wood composite member has light transmittance,ranging approximately from 80% to 95%, and an optical haze rangingapproximately from 80% to 100% in the visible light wavelength rangefrom 400 nm to 1100 nm. The optical properties depend, although notexclusively, on the choice of the infiltrating materials.

The subject wood-based light transmitting system may form anoptoelectronic system with an advanced light management, including atleast one of photonic systems, solar cells, photo-detectors, displays,and wide-angle lighting systems.

When used in a solar cell which includes an optically active layer, thetransparent wood composite member (shaped as a thin layer having athickness ranging between 100 μm and 3 mm) is disposed in opticalcontact with the optically active layer. Light incident onto thetransparent wood composite member is scattered along the light paththerethrough prior to reaching the optically active layer in the solarcell.

Alternatively, the subject wood-based light transmitting system may beused as a light-harvesting building structure having a high mechanicalstrength with a fracture strength of 23.5-45 MPa and high ductility ofthe fabricated transparent wood composite member. The mechanicalproperties depend, although not exclusively, on the choice of theinfiltrating materials.

In an additional aspect, the present invention is directed to a methodof fabrication of wood based light transmitting systems which comprisesthe steps of:

fabricating a transparent wood composite member by:

(a) pre-cutting a wood block from a natural wood in a pre-determinedangular relationship to natural internal channels of the natural wood,where the natural internal channels have walls formed fromcellulose-containing material and filled with lignin,

(b) substantially completely removing the lignin from natural internalchannels of the wood block, thus forming the lignin-devoid wood block,and

(c) upon the lignin removal, infiltrating the natural internal channelsin the lignin-devoid wood block with a filling polymer having refractiveindex substantially matching a refractive index of thecellulose-containing material of the internal channels' walls.

In the step (b), the subject fabrication process assumes:

preparing a lignin removal solution by mixing a solution of NaOH havinga concentration of 2.5 mol/L in deionized water, and a solution ofNa₂SO₃ having a concentration of 0.4 mol/L in deionized water;

immersing and boiling the pre-cut wood block in the lignin removalsolution for approximately 12 hours,

rinsing the pre-cut wood block in hot distilled water,

immersing and boiling (avoiding stirring) the rinsed pre-cut wood blockin a bleaching solution containing 2.5 mol/L of H₂O₂ in distilled wateruntil a color of the pre-cut wood block disappears, thus obtaining alignin-devoid wood block,

rinsing the colorless lignin-devoid wood block with cold water, and

preserving the colorless lignin-devoid wood block in ethanol solvent.Other lignin removal chemicals (widely used in paper making industry)include, but not limited to NaOH+Na₂SO₄ (boil) (+H₂O₂), NaClO, H₂O₂,NaClO₂+Acetic Acid, NaOH (+H₂O₂), NaOH+Na₂S (+H₂O₂), Na₂S₂O₄+ZnS₂O₄,ClO₂, CH₃COOOH, H₂SO₅, CH₃COOOH+H₂SO₅.

In the step (c), the subject fabrication process further includes:

immersing the lignin-devoid wood block in the filing polymer in theliquid phase thereof,

degassing the liquid filing polymer under pressure of approximately 200Pa for approximately 5-10 minutes to remove a gas and ethanol solventfrom the lignin-devoid wood block,

applying the atmosphere pressure to the liquid filling polymer topromote the internal channels infiltration process,

repeating the atmosphere pressure application a predetermined number oftimes, thus obtaining the polymer infiltrated wood block immersed in theliquid filling polymer,

maintaining the polymer infiltrated wood block in the filling polymerundisturbed at approximately 30° C.-60° C. for approximately 12 hoursuntil the liquid filling polymer solidifies, and

peeling the polymer infiltrated wood block from the solidified fillingpolymer, thus obtaining the transparent wood composite member.

The subject method further contemplates the steps of:

placing the transparent wood composite member on an optically activelayer of a solar cell in contagious contact therewith, thus forming asandwich structure, and

drying the sandwich structure at room temperature to firmly attach thetransparent wood composite member to the solar cell.

The subject method also contemplates the step of:

attaching the transparent wood composite member to a building at a siteof at least one window or a rooftop to serve as an energy efficientbuilding material that is capable of providing an improved thermalinsulation and daytime light harvesting.

BRIEF DESCRIPTION OF THE DRAWINGS

FIGS. 1A-1J illustrate schematically the manufacturing process of twotypes of wood blocks, where FIG. 1A depicts wood blocks pre-cut incross-direction (R-wood) and longitudinal direction (L-wood); FIGS. 1Band 1C show the internal channels in R-wood and L-wood, respectively,FIGS. 1D and 1E show SEM images of the internal channels in R-wood andL-wood, respectively, FIG. 1F depicts schematically the lignin removalstage of the subject fabrication process, FIGS. 1G and 1H depicts thecolor change of the wood block during the lignin removal (Processes I)and bleaching routine (Process II) respectively, and FIGS. 1I and 1J arediagrams corresponding to lignin content vs. time in processes I and II,respectively;

FIGS. 2A-2E show schematically the subject process of fabrication of thecross-cut transparent wood composite, where FIG. 2A shows a naturalwood, FIG. 2B shows a pre-cut slice of the natural wood, FIG. 2C shows(on a somewhat enlarged scale) a lignin-filled wood block cut out in thecrossing direction relative to the internal channels from the wood blockof FIG. 2B, FIG. 2D shows the wood slice of FIG. 2C, where lignin isremoved, FIG. 2E shows the step of polymer infiltration, FIG. 2F showsthe transparent wood block of FIG. 2D with the polymer infiltrating thenatural inner channels of the wood, and FIG. 2G shows the transparentwood composite;

FIGS. 3A-3D show Scanned Electron Microscope (SEM) images, where FIG. 3Ais an SEM image of the lignin-devoid wood block, FIG. 3B is a zoom-inlignin-devoid SEM image of FIG. 3A, FIG. 3C is an SEM image of thealigned cellulose nanofibers, and FIG. 3D is an SEM image of a polymerfilled wood composite;

FIGS. 4A-4L illustrate comparison between the transparent wood compositeprecut in cross direction (R-wood) to the inner channels and inlongitudinal direction (L-wood) along the inner channel, where FIGS. 4Aand 4B show the transmittance measurements setups with two differentanisotropic structures (cross and longitudinal directions) of thetransparent wood, respectively, FIGS. 4C and 4D are photo images of thescattered light spot for R-wood and L-wood, respectively, FIGS. 4E and4F are diagrams representative of the intensity distribution in X and Ydirections in correspondence to FIGS. 4C and 4D, respectively, FIGS. 4Gand 4H show the transparent R-wood sample placed directly on the gridsand 5 mm above the grids, respectively, FIGS. 4I and 4J show the L-woodsample placed directly on the grids and 5 mm above the grids,respectively, FIG. 4K is a diagram representative of the totaltransmittance for the natural R-wood, natural L-wood, transparent R-woodand transparent L-wood, respectively, and FIG. 4L is a diagramrepresentative of the optical haze of the R- and L-wood composites,respectively;

FIGS. 5A-5H are representative of comparison of structuralcharacteristics of the R-wood and L-wood, where FIGS. 5A and 5Billustrate mechanical forces applied to transparent R-wood andtransparent L-wood members, respectively, FIG. 5C is a diagramrepresentative of the experimental stress-strain curves for the naturalR-wood and transparent R-wood samples, respectively, FIG. 5D is adiagram representative of the experimental stress-strain curves fornatural L-wood and transparent L-wood samples, respectively, FIGS. 5Eand 5F show SEM images of cross section of the natural R-wood andnatural L-wood after the fracture of the stress-strain test,respectively, and FIGS. 5G and 5H show the SEM image of the crosssection of the transparent R-wood composite and transparent L-woodcomposite samples, respectively, after the fracture of the stress-straintest;

FIGS. 6A-6F show scanning electron microscope (SEM) images of the woodtransformation during the fabrication process, where FIG. 6A is a crosssection of the wood block showing open channels filled with lignin, FIG.6B is an SEM image of the cross section of the wood slice where thelignin is removed (the resulting white wood is shown as an inset), FIG.6C shows a cross section of the wood slice where the lignin is replacedby the refractive index matching polymer (the resulting transparent woodshown as an inset), FIG. 6D is a side view of the untreated R-woodhighlighting the alignment of the internal microchannels, FIG. 6E is anSEM image showing the densely aligned CNF in the cell wall of themicrofibers, and FIG. 6F is an SEM image showing the fibers of FIG. 6E,which have been randomly distributed after further processing to have ahigher contrast showing the aligned fibers broken down into a randomlydistributed network;

FIGS. 7A-7C show the comparison of transmittance between the fabricatedtransparent wood composite and natural wood, where FIG. 7A shows anon-transparent wood block, FIG. 7B shows a transparent wood compositemember with a thickness of 1 mm, and FIG. 7C is a diagram representativeof comparison of the total defused transmittance of the regional naturalwood and the transparent wood composite;

FIG. 8A is a diagram representative of the transmittance characteristicof wood slabs with different percentage of lignin removed, FIGS. 8B and8C are fluorescent images of the wood on different scale, FIGS. 8D-8Gare fluorescent images of the wood with different weight percentage oflignin removal level;

FIGS. 9A-9B demonstrate the haze characteristics of the transparent woodcomposite sample and its application to GaAs solar cell, where FIG. 9Ashows the light scattering by the transparent wood composite, FIG. 9B isa diagram representative of the transmittance and haze, respectively, ofthe transparent wood composite sample used in the experiment, FIG. 9C isa schematic representation of distribution of the light incident on asolar cell using the wood transparent composite attached to the activelayer, and FIG. 9D is a diagram representative of the current densityvs. voltage characteristics for the bare GaAs cell and the GaAs cellwith the light management transparent wood composite coating,respectively;

FIGS. 10A-10B show schematically an edifice with a transparent wood rooftop (FIG. 10A) and a building material (FIG. 10B) made from the subjecttransparent wood composite which scatters the transmitted light in theforward direction to create substantially uniform lighting and iscapable of reducing the conductive heat flow to maintain a substantiallyconstant internal temperature;

FIG. 11A is a SEM image of the transparent wood microstructure, FIG. 11Bis a top view of the guided light propagation in a thick transparentwood composite block, FIG. 11C is a diagram representative of a hightransmittance and low reflectance with effective broadband forwardscattering in the visible wavelength range through the 0.5 cm thicktransparent wood window, FIG. 11D shows the transmitted beam pattern ofa 45° laser beam incident on the transparent wood composite sample, andFIG. 11E shows a diagram of the intensity of the light vs. thescattering angle in X and Y directions;

FIG. 12A is a diagram representative of the transmittance percentage vs.haze percentage of standard glass, transparent paper, and the subjecttransparent wood composite, respectively, FIG. 12B is a photographicevidence of the problematic glaring effect with glass in comparison withthe uniform and comfortable lighting through the transparent woodcomposite, FIG. 12C is photographic evidence of the uniform lightdistribution inside the house model when using the subject transparentwood composite as daylight harvesting roof top in comparison with glass,and FIG. 12D is a diagram representative of the light intensitydistribution of the glass roof vs. the transparent wood composite roof;

FIG. 13A is an illustration of the radial and axial heat transport inthe transparent wood composite, and FIG. 13B is a diagram of themeasured thermal conductivities of the standard glass, epoxy, axial andradial directions of the transparent wood, respectively; and

FIG. 14A depicts an impact test of a piece of standard glass incomparison with the transparent wood composite of the similar thickness,FIG. 14B depicts diagrams of the strain-stress curves of the transparentwood composite and glass, respectively, and FIG. 14C is photographicevidence that the transparent wood composite sample is water resistantand exhibits no obvious change after 72 hours immersion in water.

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENT

Referring to FIGS. 1A-1H and 2A-2G, the transparent wood composites werefabricated by efficient and simple process including removing the lightabsorptive lignin to form the lignin-devoid wood block (in the firstmanufacturing stage best shown in FIGS. 1F and 2D), and backfilling thenano/microsized channels in the lignin-devoid wood block withindex-matching polymers (in the subsequent second manufacturing stagebest shown in FIG. 2E). By filling the channels with the properlyselected polymer(s), the refractive index (RI) mismatch can be greatlyreduced and the light reflection can be suppressed to increase the woodsample transparency.

The well-defined internal channels in the natural wood have a lowtortuosity, which permits rapid removal of the colored lignin depositedinside the internal channels. After lignin removal, the open internalchannels allow fast infiltration by the polymer(s) to decrease the lightscattering and increase the mechanical strength of the wood/polymercomposite.

The resulting polymer infiltrated lignin-devoid wood block demonstratesoptical transmittance of approximately 87±5% and a high optical haze ofup to 80-100% over the broad spectrum in the visible wavelength range of400 nm-1100 nm. The subject process preserves the well-alignedmicrostructure of CNF in the natural wood upon the lignin removal andthe polymer infiltration which contributes to effective incident lightscattering.

The pre-cut wood block may have various thicknesses to form a thickblock or a thin layer, for example, the thickness of the woodtransparent composite block in the subject method may range from roughly100 μm (typical paper thickness) to a millimeter, or thicker, forexample, 1.4 cm.

The subject transparent wood composites have enhanced dimensionalstability in water and humid environments depending on the fillingmaterial, which may be selected from a large group of index matchingmaterials.

When the thin layer of the subject transparent wood composite isattached to a GaAs solar cell as a light management coating, thestructure demonstrates an enhanced efficiency of the solar cell by 18%.

The subject transparent wood composite also has been demonstrated as avaluable building material which is capable to efficiently harvestsunlight to provide consistent and uniform indoor lighting. Thevertically aligned transparent wood fibers in natural wood exhibit anefficient visible light guiding effect with a large forward to backscattering ratio. When used as a window or a rooftop, the subjecttransparent wood effectively guides sunlight into the building. Uniqueoptical properties, such as an extreme optical haze (>95%) in thebroadband range and a high transmittance (>85%), lead to a uniform andcomfortable indoor ambient lighting without a glare effect in buildings.The transparent wood composite also has better thermal insulation thanglass with at least three times lower thermal conductivity. Greenhousegas emission from residential and commercial sectors can mainly beattributed to the energy use of buildings which is reduced by the use ofthe transparent wood. The application of the subject energy efficienttransparent wood building material can yield substantial energy savingswith associated reductions in greenhouse gas emission. The wood basedtransparent composites can find a range of potential applications in thenext-generation energy efficient buildings.

As shown in FIGS. 1A-1H and 2A-2G, the subject fabrication processbegins with pre-cutting a wood block 10 from a natural wood 12 in apredetermined angular relationship to a direction of natural internalchannels 14 extending along the trunk of the natural wood 12. The woodblock 10 has two opposing planes, for example, upper and bottom cutplanes 16 and 18, respectively, which may be parallel or non-paralleleach to the other.

The angular relationship between the cut planes 16, 18 and the directionof the internal channels 14 may encompass any angular displacementbetween the direction of the internal channels 14 and the cut plane ofthe wood block 10. However, for the sake of clarity, as an example only,but not to limit the scope of protection of the subject invention, twoangular relationships are presented in following paragraphs, including(1) cross sectional cut forming radial (R-wood) type of the wood block10, shown in FIGS. 1A-1B, 1D, 2C-2D, and 2F, and (2) longitudinal cutforming longitudinal (L-wood) wood block 10 as shown in FIGS. 1A, 1C,and 1E. As indicated supra, other angular relationships for cutting thewood block 10 ranging between R (90°) and longitudinal direction (0 or180°) are also contemplated in the process of fabrication of the subjecttransparent wood composite structure.

The channels 14 in the wood are naturally filled with lignin 20. Thechannels 14 have walls 22 which are made from cellulose and/orhemicellulose. The subject fabrication method contemplates the step oflignin removal, which is schematically shown in FIGS. 1F and 2D, wherewood blocks are heated in water with chemicals NaOH and Na₂So₃ inProcess I, and bleached in solution of H₂O₂ in water in Process II, bothof which combinably constitute the lignin removal stage to form alignin-devoid wood sample 24.

As shown in FIGS. 1G-1H, during lignin removal in the Processes I andII, the brown and yellowish wood block 10 gradually becomes lighter, andfinally becomes white (white wood 24) due to the light scattering andthe absence of light absorption by lignin. Upon complete lignin removal,the channels in the white wood block 24 are open to permit thesubsequent infiltration of the lignin-devoid sample 24 with a refractiveindex-matching polymer to decrease the light scattering as shown in FIG.2F-2G, thus forming a highly transparent wood composite 26.

The subject process is applied to fabrication of R-wood, as well asL-wood, transparent composites, although with different process regimesdue to the fact that in the R-wood type, the internal channels 14 areshorter and their openings 30 allow easier access by the chemicals (asshown in FIGS. 1B, 1D) than in L-wood where the longer open channels 14extend in the direction of planes 16, 18 of the wood block 10 (as shownin FIGS. 1C and 1E). The processes of lignin removal and polymerinfiltration are longer for the L-wood than for the R-wood.

The difference in the channels length in R-wood and L-wood dictates thedifference in lignin-renewal and polymer-infiltration kinetics. Due tothe fact that the channels lengths in the R-wood (FIGS. 1B and 1D) ismuch shorter than the lengths in L-wood (FIGS. 1C and 1E), the ligninremoval and polymer infiltration faster in the R-wood slices.

There are a large number of straight channels 14 in a wood trunk 12extending along the growth direction. Wood slabs 10 with dramaticallydifferent microstructures can be readily obtained by cutting indifferent directions, for example, as shown in FIG. 1A, R-wood has openchannels 14 perpendicular to the plane(s) 16 and 18 with a depthsubstantially the same as the thickness of the wood. L-wood has theidentical mesoporous open channels but the depth of the open channelscorresponds to the length of the wood block 10. The difference in theanisotropic microstructure of R- and L-wood leads to a significantdifference in lignin removal rate, where lignin can be extracted mucheasier in R-wood due to the open channels with a short depth (FIGS. 1Band 1D). It takes a longer time to extract lignin from L-wood (FIGS. 1Cand 1E).

As shown in FIGS. 1D and 1F, which display the microstructures of R-woodand L-wood, respectively, the open channels in wood are not uniform indiameter, ranging from 10 μm to 80 μm, which leads to the difficulty inremoving lignin in L-wood.

In an experiment, wood blocks 10 with a dimension of 50 mm by 50 mm anda thickness of 3 mm were used. As shown in FIG. 1F, pre-cut wood blocks10 were soaked in boiling solution containing NaOH and Na₂SO₃ (ProcessI) to dissolve part of the lignin content. Then the wood blocks weresubsequently transferred into H₂O₂ solution in water for bleaching,i.e., to remove the remaining lignin (Process II). The lignin removingchemical is typically prepared as described in precious paragraphs.

Other lignin removal chemicals (widely used in paper making industry)include, but not limited to NaOH+Na₂SO₄ (boil) (+H₂O₂), NaClO, H₂O₂,NaClO₂+Acetic Acid, NaOH (+H₂O₂), NaOH+Na₂S (+H₂O₂), Na₂S₂O₄+ZnS₂O₄,ClO₂, CH₃COOOH, H₂SO₅, CH₃COOOH+H₂SO₅.

Since lignin is colored and cellulose is colorless, the color of thewood blocks indicates the amount of lignin remaining in the wood blocksurface. The color comparison for lignin removal in R-wood and L-wood isshown in FIGS. 1G and 1H. The color becomes lighter as lignin is beingremoved (identified by the duration of the process). As can be seen inFIGS. 1G-1H, the process for R-wood is much faster than that for L-woodevidenced by lighter samples of R-wood than the L-woods at the sameprocess duration. The experimental setup, shown in FIG. 1B, can bescaled up to process a number of wood blocks at the same time for massmanufacturing.

The lignin removal was quantified in both types of woods (FIGS. 1I and1J), where the y-axis corresponds to the lignin content of the woodblocks after a certain period of time for the lignin removal processes Iand II. In both types of wood (R-wood and L-wood) treated in the ProcessI, the lignin was removed rapidly in the first hour, where the ligninloss for R-wood is higher, up to ˜25%. FIG. 1I shows clear differencesin lignin removal kinetics, where the process is much faster in R-woodthan in L-wood. During the Process II, the lignin in R-wood is alsoremoved rapidly (FIG. 1J). These results agree with the fact that thechannel length is much larger in L-wood than that in R-wood.

The anisotropic open channels 14 in the wood blocks not only allow fastlignin removal but also lead to fast polymer infiltration to form atransparent wood composite, especially for R-wood. FIG. 3A shows the SEMimage of a wood block after most of lignin has been removed. The openchannels are made of cellulose and hemicellulose. The wood blockdisplays massive open channels and openings along the wood growthdirection. Zoomed-in SEM (shown in FIG. 3B) also shows the smaller,secondary holes which enables material transport in the radialdirections in the wood trunk. The inset of FIG. 3B shows thelignin-devoid white wood block 24. The microstructures with thewell-defined channels are well preserved during the lignin removalprocess, which is important for the rapid infiltration of the polymer.Zoom-in SEM image also shows the cellulose nanofibers on the cell walls22, which are aligned and densely packed as shown in FIG. 3C.

Subsequent to the lignin removal, a filling polymer is infiltrated intothe wood microstructures under vacuum assistance, as shown in FIG. 2E.The filling polymer 28 may be a material from a group of material havinga refractive index close to the refractive index of cellulose (˜1.48).The filling polymer may be selected from, but not limited to, the groupof materials, including:

Thermosetting polymers, such as, for example, Polyester fiberglass,Polyurethanes polymers, Vulcanized rubber, Bakelite, Duroplast,Urea-formaldehyde, Melamine resin, Diallyl-phthalate (DAP), Polyimidesand Bismaleimides, Cyanate esters or polycyanurates, Furan resins,Polyester resins, Silicone resins, Benzoxazine resins, Bis-Maleimides(BMI), Cyanate ester resins, Epoxy (Epoxide) resins, Phenolic (PF)resins, Polyester resins, Polyimides, Polyurethane (PUR) resins,Silicone Resins, Vinyl ester resins,

Thermoplastic polymers, such as, for example, Acrylic, ABS, Nylon, PLA,Polybenzimidazole, Polycarbonate, Polyether sulfone, Polyetheretherketone, Polyetherimide, Polyethylene, Polyphenylene oxide, Polyphenylenesulfide, Polypropylene, Polystyrene, Polyvinyl chloride, Teflon,

Cellulose derivatives, such as, for example, Cellulose acetate,Cellulose acetate butyrate, Cellulose triacetate, Methyl cellulose,Hydroxypropyl methyl cellulose, Ethyl cellulose, Hydroxyethyl cellulose,Carboxymethyl cellulose, Dissolved cellulose, Nanofibrillated cellulose,Cellulose nanocrystals,

functional index matching materials, such as, for example, liquidcrystal, pressure/temperature sensing materials, piezoelectricmaterials,

as well as colorless polymer nano-glue, transparent liquid epoxy resinprecursor with low viscosity, a mixture of a resin and non-blushingcycloaliphatic hardener, polyvinylpyrrolidone (PVP), Poly(methylmethacrylate) (PMMA), Poly(vinyl alcohol) (PVA), Polydimethylsiloxane(PDMS), etc. As one of numerous examples, transparent liquid epoxy resinprecursor (the mixture of #300 resin and #21 non-blushing cycloaliphatichardener) with relatively low viscosity, can be used.

For the polymer filling stage of the subject fabrication process, thelignin-devoid white wood sample 24 is immersed in the liquid polymer 28followed by repeated cycles of vacuum/de-vacuum processing, asschematically shown in FIG. 2E. A complete infiltration is achievedafter about three vacuum/de-vacuum cycles. SEM image in FIG. 3D andschematics in FIG. 2F show that the filling polymer fully infiltratesthe channels and apertures. The original cellulose walls of woodchannels and the infiltrating polymer can be clearly distinguished inthe SEM image in FIG. 3D. Full infiltration was confirmed in theexperiments by breaking the wood-polymer composite in the middlefollowed by the SEM imaging. The polymer infiltration process does notdestroy the frameworks of the natural wood microstructures. Stronginteraction (such as the hydrogen bonding or Van Der Waals forces)between the wood cellulose and infiltrating polymer(s) preserves theframework of the wood micro-structures and prevents from beingstructurally altered. After the polymer infiltration, the white woodblock 24 (inset in FIG. 3B) becomes optically clear, and thus, the thick(up to a centimeter) piece of wood becomes a highly transparentstructural material 26 as shown in FIGS. 2F-2G and 3D.

The anisotropic structures in the two types of transparent wood (R-woodand L-wood) potentially lead to a range of anisotropic properties. Theanisotropic optical properties of R-wood and L-wood have been thoroughlyinvestigated. FIGS. 4A-4L illustrate the optical measurement for the twotypes (R-wood and L-wood) of transparent wood composites 26,respectively. The thickness of both R-wood and L-wood for the study was2 millimeter for comparison purpose.

The transmittance measurement setups for transparent R-wood andtransparent L-wood composite samples 26 are shown in FIGS. 4A and 4B,respectively. A 532 nm single mode laser (from Thorlabs, Inc) was usedas the incoming light source for the anisotropy measurements. The laserwas collimated first with a spot size around 200 μm beforeperpendicularly illuminating the transparent wood samples. The incominglight rapidly diverges due to the scattering in transparent woodcomposites. While the scattering effect is isotropic in the lightpropagation cross-section plane for R-wood (as shown in FIG. 4C), thelight scattering in L-wood is highly anisotropic (as shown in FIG. 4D).

A photodiode power sensor S130C from Thorlabs, Inc. was used to recordthe scattered light intensity distribution in both the x and y directionin R-type and L-type (marked in FIGS. 4E and 4F, respectively) of the2-dimensional plane perpendicular to the light propagation direction (zdirection).

After the polymer infiltration of the R-wood, the index mismatch betweenthe filling polymer and the cellulose fibers facilitatesangle-independent scattering of the single mode Gaussian laser beam, asshown in FIG. 4E. The resulting scattered light thus exhibits aGaussian-like distribution with similar scattering angle in both the xand y directions.

On the other hand, the wood fibers in the L-wood are aligned in the xdirection, yielding a discrete index variation in y direction (as shownin FIG. 4F). A greatly traversely-expanded beam is observed in the ydirection with an extremely large scattering angle, which resulted froma strong light diffraction by densely packed and aligned wood fibers. Inthe x direction, there is little refractive index fluctuation, and theincident light is scattered slightly, which resulted in a mild lightspace distribution. The strong anisotropic microstructures in L-woodlead to its intense anisotropic optical properties.

The anisotropic transparent wood composites also exhibit unique imagingeffects. A grid 32 with perpendicular and parallel lines was designed toshow the angle dependence in light scattering (as shown in FIGS. 4G-4J).The grid lines can be clearly seen for both the transparent R- andL-wood transparent composite samples 26 when in contact with the surfaceof grid lines (FIGS. 4G and 4I). However, the visual effect is differentwhen the transparent wood composites 26 positioned 5 millimeters abovethe lines. For the R-wood (FIG. 4H), no lines can be observed due to thehigh transmittance haze. For the L-wood (FIG. 4J), in sharp contrast,the grid lines are turned to parallel lines, while the lines parallel tothe open channels diminish, which is in accordance with the anisotropichaze effect shown in FIG. 4F.

In addition to the anisotropic behavior of optical properties, the totaltransmittance and optical transmittance haze of the transparent woodcomposite has also been studied (FIGS. 4K and 4L). An integrated spherewas used to measure the optical transmittance and transmittance haze.Natural L- and R-wood show almost negligible transmittance due to thestrong lignin absorption. After lignin extraction and polymerinfiltration, both types of transparent wood composites showdramatically high transmittance as illustrated in FIG. 4K. The measuredR-wood transmittance reaches up to 90%, higher than in the L-wood, whichis due to better filling of the polymer due to the small depth for theopen channels in R-wood.

Both transparent wood composite samples (R-wood and L-wood) exhibitlarge haze covering the entire visible wavelength ranging from 400 nm to800 nm, while R-wood shows a generally higher value than that of L-wood(FIG. 4L).

For the transparent wood composite where the interface between themicrosized cellulose and the polymer has a roughness larger than thewavelength of the incoming light, the scattering intensity issubstantially independent of the wavelength. This broad range lightmanagement is referred to as Mie scattering. The high optical haze asexhibited by both types of wood can be potentially used for a wide rangeof optoelectronics applications where advanced light management isneeded to improve the light coupling and extraction efficiency needed insolar cells and displays.

The unique mesostructures in transparent wood composites not only leadto anisotropic optical properties, but also to dramatically anisotropicmechanical properties in different directions. In order to carry out theexperiment, transparent R-wood samples 26 and transparent L-wood samples26 were fabricated having a shape shown in FIGS. 5A-5B, respectively,with dimensions of about 50 millimeters long, 10 millimeters wide and 3millimeters thick for mechanical tests. The samples 26 are shown inFIGS. 5A-5B with channels 14 extending in crossing and longitudinaldirections, respectively, between the cut planes 16, 18. A Tinius OlsenH5KT tester was used to carry out the stress-strain measurement for thesamples. Natural wood samples cut into similar dimensions were alsoevaluated for comparison.

Compared with the natural R-wood, the transparent wood composite showsan improved mechanical strength, with a fracture strength up to 23.5 MPa(as presented on the diagram in FIG. 5C). For comparison, the fracturestrength of the natural R-wood is only 4.5 MPa. Lignin removal andpolymer filling lead to the transparent R-wood material with improvedstrength (FIG. 5C). An additional benefit is that the transparent woodcomposite possesses a ductility similar to the nature wood, i.e., ˜3.7%.

The transparent L-wood has a fracture strength of about 45 MPa, around 2times higher than that of transparent R-wood (as presented in thediagram in FIG. 5D). Transparent L-wood also has a higher ductility thanthe transparent R-wood. Compared with the natural L-wood, thetransparent L-wood after polymer infiltration has a higher both strengthand ductility.

In most materials, mechanical strength and ductility are mutuallyexclusive. Simultaneous increase of the strength and ductility isabnormal but highly desired for structural applications. Compared withthe natural wood, the increase in both ductility and mechanical strengthleads to a much higher toughness in transparent wood composites, makingthe subject transparent wood highly desirable for structural materialapplications.

The cross section after the stress-strain test until fracture occurredhas been studied by the inventors. The open channels in the natural woodare visible in the SEM images, shown in FIGS. 5E and 5F. The forceapplied is perpendicular relative the open channels in the R-wood, andis parallel to the channels in the L-wood.

While the SEM images show similar morphology after breaking, themacroscopic features shown in the photographs after the fracture arehighly different. The cross section of the L-wood has a rough surface(inset of FIG. 5F) where the breaking happens within the channels.Meanwhile, the R-wood shows a cross section more like a brittle material(small surface, inset of FIG. 5E), due to the parallel stacking of thechannels where breaks occur between the channels.

Micro-scale and macro-scale studies have been performed for thetransparent wood composites to investigate failure mechanisms. In thetransparent R-wood and L-wood samples, the macroscopic structures of thecross section after mechanical fracture are similar, with a smoothinterface. In transparent wood composites, the filling polymers becomecross-linked with the cellulose backbone after the lignin removalforming a three-dimensional network. The alignment structure observed inthe SEM leads to higher mechanical strength in the transparent L-woodthan in the transparent R-wood.

FIGS. 5G and 5H show cross section SEM of the transparent R-wood andtransparent L-wood, respectively, after fracture in the stress-straintest. The inset in FIGS. 5E-5H is the photo of the samples aftermechanical fracture.

Two types of anisotropic wood composites have been manufactured bytaking advantage of the unique natural macrostructures in natural wood.In both transparent R-wood and L-wood composites, two stages in sequencewere used to fabricate anisotropic transparent wood composites: (1)lignin removal from the open channels, and (2) polymer infiltration intothe open channels. The well-defined, aligned channels largely facilitatethe two processes. In both types of the transparent wood, the naturecellulose structures are well preserved, the colour of lignin isremoved, and the porous structure is filled with a polymer, which leadsto a high transmittance of up to 90%.

The numerous polymer-cellulose interfaces support the forward lightscattering, which leads to a high optical haze at the same time. The twotypes of transparent wood composites (R-wood and L-wood) also displaydifferent light scattering and mechanical properties. For example, thetransparent wood with open channels in the plane (L-wood) ismechanically stronger and tougher than the plane (R-wood).

Materials and Chemicals.

Basswood from Walnut Hollow Company was used for experiments. Thechemicals used in removing lignin contents from wood were sodiumhydroxide (>98%, Sigma-Aldrich), sodium sulphite (>98%, Sigma-Aldrich)and hydrogen peroxide (30% solution, EMD Millipore Corporation). Thepolymer used for infiltration was Epoxy Resin (#300 resin and #21 nonblushing cycloaliphatic hardener, AeroMarine Products, Inc.).Alternatively, Polyvinylpyrrolidone (PVP, average M_(w)˜1,300,000,Sigma-Aldrich) was used as the filling polymer in the lignin-devoidwood. The solvents used were ethanol alcohol (190 proof, 95%,Pharmco-Aaper) and deionized (DI) water.

Lignin Removal from Wood.

The lignin removal solution was prepared by dissolving NaOH and Na₂SO₃in deionized (DI) water resulting in a concentration of 2.5 mol/L and0.4 mol/L, respectively. The wood slices were immersed in the ligninremoval solution and boiled for 12 hours, as shown in FIG. 1F (ProcessI), followed by rinsing in hot distilled water three times to removemost of the chemicals. The wood blocks were subsequently placed in thebleaching solution (H₂O₂, 2.5 mol/L in DI water) and boiled withoutstirring (Process II shown in FIG. 1F). When the yellow color of thesamples disappeared, the samples were removed and rinsed with coldwater. The lignin-devoid samples were then preserved in ethanol

Polymer Infiltration.

Epoxy Resin was prepared by mixing the two liquid components (#300 resinand #21 non blushing cycloaliphatic hardener) at a ratio of 2 to 1. Thelignin-devoid wood samples were placed at the bottom of a dish andimmersed in the liquid resin. The solution was then degassed (vacuum)under 200 Pa to remove the gas and ethanol solvent in wood as shown inFIG. 2E. In approximately 5 minutes, the vacuum was released to allowthe polymer filling into the wood structure at atmosphere pressure. Theprocess vacuum/de-vacuum was repeated for 3 times as shown in FIG. 2E.All these processes were terminated within 30 minutes to avoid thepolymer solidification. Finally, the dish containing the wood sample andpolymer was kept static (undisturbed) at 30° C. for 12 hours. Thepolymer-infiltrated wood sample was peeled from the dish after thepolymer was completely solidified.

When using Polyvinylpyrrolidone (PVP), the polymer was dissolved inethanol at a concentration of 15% by mass of polymer. After fulldissolution, the lignin-devoid wood was placed at the bottom of a dishand immersed in a PVP solution. The solution depth was approximately anorder of magnitude greater than the wood thickness. The solution wasthen degased under 200 Pa for approximately 10 minutes to ensure fullinfiltration. Finally, the dish was placed on a hot plate at 60° C. Thepolymer-infiltrated wood sample was peeled from the bottom of the dishafter the solvent was completely evaporated.

Measurements and Characterizations.

The morphologies of the transparent wood composite were characterized bya scanning electron microscope (SEM, Hitachi SU-70). The transmittancespectrum and haze were measured with a UV-Vis Spectrometer Lambda 35(PerkInElmer, USA.). The lignin contents were measured by the standardmethods for lignin determination (Technical Association of Pulp andPaper Industry Standard Method T 222-om-83).

About 1 g (m₀) of dry wood was measured and extracted with ethanolalcohol for 4 hrs, which was then treated with 15 mL of cold H₂SO₄ (72%)for 2 hrs with vigorous stirring at 20° C. The mixtures were transferredto a beaker and diluted to 3% by mass of H₂SO₄ by adding 560 mL of DIwater, and boiled for 4 hrs. After cooling down, they were filtered andwashed with DI water. The insoluble materials were dried and weighed(m₁). The lignin content was calculated as: [m₁/m₀]×100%.

Photocurrent-voltage characteristics of solar cells were monitored witha voltage-current source meter (2400 Keithley) illuminated by an OrelSolar Simulator (AM 1.5, 100 mW/cm⁻²) with a scan rate of 10 mV/s.

A 532 nm single mode laser DJ532-10 (Thorlabs Inc.) was used as theincoming light source with stabilized output power. The laser wascollimated first with a spot size around 200 μm before perpendicularlyilluminating the samples. The Gaussian beam quickly diverges afterpropagating through the transparent wood composite. In order to map thescattering distribution, a photodiode power sensor S130C from Thorlabswas used to record the scattered light distribution in the 2-dimensionalplane perpendicular to the light propagation direction. A pinhole with aconstant diameter of 5 mm was placed in front of the photo diode torecord the light power at various angles along x and y directions. Themechanical properties were structured using a Tinius Olsen HSKT testingmachine. The wood was selected without joints or fasteners with adimension of about 50 mm×10 mm×3 mm

Anisotropic channels are found within the trunks of most trees. Thesemesoporous channels have a diameter of 25 μm to 50 μm and allow theextraction of lignin along the channel direction. Dry basswood slabs oftypical thickness in the range of 100 μm to 14 mm were obtained bycutting perpendicular to the tree growth direction. The open channels inthe thin sections enable the fast removal of lignin from the wood slab.

Generally, basswood contains 18% to 21% lignin and 79% to 82%hemicellulose and cellulose by mass. The SEM image of the sample 10before lignin 20 removal is shown in FIG. 6A. Lignin 20 was extractedfrom the channels 14 and apertures 30 by the chemical processes of NaOHtreatment followed by H₂O₂ bleaching. During removal of the coloredlignin, the wood slab 10 gradually lost its color (as the lignin wasbeing removed), and became visibly whiter due to the large lightreflection at the interfaces. During the lignin removal process, it wasobserved that the brown color of the natural wood diminished graduallyand finally changed to snow white (the inset 24 shown in FIG. 6B).

By controlling the speed and duration of the lignin removal process, thelignin was removed almost entirely while simultaneously preserving themicrostructure of the wood, as shown in FIGS. 6B and 6D. It isnoteworthy that well-aligned microstructure in the wood block wassuccessfully preserved after the lignin removal process (FIGS. 6E and6F).

The open vertical channels after lignin removal also support the rapidinfiltration of filling materials to achieve other functionalities.Index-matching polymers 28 were used to fill the lignin-devoid channels14 and to reduce light scattering to attain high optical transmittance.

As an example, Polyvinylpyrrolidone (PVP) 28 was selected (from a numberof different refractive index matching polymers) as the filling materialbecause of its excellent transparency, relatively low viscosity inethanol and good wettability on cellulose. These characteristics of PVPenabled it to fully permeate the micro-scale apertures 30 in wood. PVPis environmentally friendly and biodegradable, similar to woodnanofibers.

FIG. 6C shows a scanning electron microscope (SEM) image of the woodblock after PVP polymer filling, where all the apertures 30 and channels14 between the cellulose walls have been completely filled with polymer28. The resulting transparent wood 26 is shown as an inset in FIG. 6Cwith well-preserved wood textures.

In addition to functioning as an index matching material with theremaining cellulose, the polymer infiltration into the woodmicrostructures also mechanically glues the wood cellulose nanofibers(CNF) together.

The subject method for fabricating transparent wood composites allowsthe original alignment of CNF to be preserved.

FIG. 6D illustrates the cell walls of the channels 14 and apertures 30.The CNF are clearly aligned without being affected (as shown in FIG.6E). These aligned CNF lead to enhanced mechanical properties. The closepacking of the aligned CNF complicates their characterization by SEM.The snow-white wood 24 has been further disintegrated for a longerperiod of time to achieve satisfactory SEM with a higher contrast. Uponfurther processing, the aligned fibers were broken down into a randomlydistributed network as is depicted in FIG. 6F.

After the lignin is removed, wood exhibits a bright white color with alow overall transparency, which is due to the large refractive indexdifference between cellulose and air. PVP is a highly transparentpolymer with a refractive index of about 1.53 at wavelength λ=550 nm, avalue close to the refractive index of ˜1.48 for cellulose. Thereflection of light normal to the interface is 0.04% for cellulose andPVP (RI=1.48 and RI=1.53, respectively), compared to 4.4% for air andcellulose (RI=1.00 and RI=1.48, respectively). Consequently, after PVPinfiltration, light reflection and scattering along the woodmicrochannels have been greatly suppressed but not completelyeliminated. Additionally, high transparency is attained along with highhaze. The optical properties can be more finely tuned with polymers ofdifferent refractive indexes.

FIG. 7A shows a natural wood block 10 measuring 30 mm×22 mm×1 mm. Thewood composite 26 shown in FIG. 7B is highly transparent and the textunderneath is clearly visible. The diagram of measured transmittances ofthe transparent wood and the original wood are shown in FIG. 7C. Thenatural wood 10 in FIG. 7A shows zero transmittance at wavelengths from400 nm to 600 nm, which is mainly due to the large light absorption ofthe lignin matrix. The maximum transmittance does not exceed 25% overthe measured range of wavelengths. In sharp contrast, the transparentwood 26 (FIG. 7B) exhibits a transmittance of about 90±5% in air fromvisible (λ=400 nm) to near infrared (λ=1100 nm). The overalltransparency for a 1 mm thick wood transparent composite is comparableto transparent glass, plastic and cellulose-based nanopaper. Inexperiments, high transparency has also been demonstrated for woodsamples as thick as 14 mm and larger. These results clearly confirm theeffectiveness of the subject process containing the step of ligninremoval followed by the step of polymer infiltration for the productionof transparent wood composites.

In order to quantify the influence of lignin content on the opticalproperties of wood, absorption measurements were performed at variousstages during the processing. FIG. 8A shows the transmittance of woodwith different amounts of lignin. Overall, the absorption edges shift tolonger wavelengths with increasing lignin content. For untreated (curveA) and 33% (line B) lignin removal (LR) samples, the absorption rangecovers the entire visible light spectrum (% LR represents lignin removalby mass). In comparison, the sample with 50% LR (curve C) and withcomplete lignin removal (curve D), exhibit a greatly enhancedtransmittance and minimized light absorption in the entire visible lightrange.

In addition, lignin exhibits fluorescence emission since it containschromophores. Photoluminescence techniques were used to verify thelignin content in the wood samples with varying amounts of lignin.Fluorescence images of wood with different weight percentage of ligninremoval (LR) level are shown in FIGS. 8D-8G, beginning from wood sample10 (in FIG. 8D) to lignin-devoid wood sample 24 (in FIG. 8G). Ingeneral, they are in agreement with the results presented in the TAPPIstandard lignin measurement method which uses luminescence to reveal thepresence of lignin. It is seen that the fluorescence intensity fororiginal wood is weaker than the 33% LR sample. This unusual behavior iscaused by the self-quenching of the fluorophores. This often happenswhen the local fluorophore concentration exceeds the quenchingconcentration. The 3D fluorescence measurements (shown in FIGS. 8B-8C)show clear morphology of the wood's microstructure.

In addition to the high diffusive transmittance, the transparent woodcomposites exhibit high transmittance haze. Light scattering by thetransparent wood composite is shown in FIG. 9A. As shown in FIG. 9B, themeasured haze is around 80±5% over the wavelength range from 400 nm to1100 nm. This is a very unique property compared to transparent glass orplastic materials with haze less than 1%. The transparent wood compositehaze is also much higher than that of the ultrahigh haze nanopaper,which exhibits a haze value of approximately 60%. The high haze intransparent wood composites originates from the unique microstructure ofwood.

Returning to FIGS. 6B and 6D, cellulose microchannels are thepredominant structural component of wood. Each microchannel is composedof microfiber units that can be further processed into nanofibers. Thereare two reasons for the measured high haze. First, the wood fibers andchannels often have micro-curvatures with bumps, microstripes andmicrocavities (FIG. 6D), causing them to function as lightreflection/scattering centers. Second, the microfibers and microchannelscan guide the incident light along the axial direction for efficientforward scattering.

The utility of transparent wood composites was demonstrated when used asa substrate for a GaAs solar cell 40 shown in FIG. 9C. In an effort toimprove the overall conversion efficiency of solar cells, lightmanagement plays an important role. The subject transparent woodcomposites, with their high transmittance and high haze serve asefficient light management coatings or substrates for optoelectronicdevices, such as photodetectors and solar cells.

The transparent wood 26 used for the solar cell 40 had a transmittanceof around 90% and a haze of about 80% over a broad wavelength range. Adrop of ethanol was deposited on the surface of the existing solar cell40. Then the transparent wood was placed on top of the cell to formcontiguous contact with the active layer 42. The sandwich structure wasallowed to dry at room temperature until the wood was firmly attached tothe surface of the bare GaAs solar cell.

The measured current density-voltage (J-V) characteristics of the solarcell are shown in FIG. 9D. The solar cell's electronic properties,including short circuit density (J_(SC)), open circuit voltage (V_(OC)),fill factor (FF, the ratio of the maximum output power solar the productof V_(OC) and J_(SC)) and the overall conversion efficiency, which areextracted from the J-V curves, are presented in Table I which reflectsthe comparison of electrical properties of GaAs solar cell before andafter the attachment of a transparent wood composite substrate.

TABLE I Voc Jsc FF Efficiency [V] [mA · cm⁻²] [%] [%] Bare GaAs cell0.952 17.10 75.1 12.21 GaAs cell with 0.968 19.78 76.0 14.41 transparentwood Enhancement [%] 0.63 15.67 1.20 18.02

An enhancement of 15.67±3% in short circuit density and a corresponding18.02±3% boost in overall conversion efficiency have been observed undera one sun illumination. This is mainly due to a combination of a forwardscattering effect and an index matching effect between air and GaAsafter attachment of the transparent wood 26 to the top surface of GaAssolar cell in optical coupling with the active layer 42.

With just a PVP coating (no wood) where the interfacial refractive indexmismatch has been suppressed, the Jsc enhancement is 10.1±3%, less thanthat of the transparent wood coating. A slight enhancement in fillfactor has also been observed, which serves as an indicator that thedark saturation current has not been degraded. The transparent woodsubstrate significantly improves the performance of a bare GaAs solarcell as a light management layer, similar to the effects observed withtransparent paper. The high transmittance allows light to reach thesurface of GaAs solar cell with less loss. The normal incident lightbecomes diffusive when it reaches the solar cell's top surface due tothe high haze. This phenomenon results in increasing the travelling pathof photons in the solar cell and improves the possibility of a photonbeing captured within the cell's active region. Additionally, thedecreased refractive index mismatch between GaAs and air after woodcoverage allows light reflection to be suppressed leading to anincreased light flux into the solar cell.

The application of the transparent wood has also been demonstrated as anenergy efficient light harvesting building material with the followingadvantages. First, the subject transparent wood can efficiently harvestsunlight with a broadband transmittance of >85%. Due to the extremelyhigh haze (≈95%) of transparent wood, the indoor illumination can bemaintained substantially uniform and consistent. Second, the transparentwood exhibits a directional forward scattering effect, which can be usedto effectively guide sunlight into the building. Third, wood cellspresent large phonon resistance with multiple boundaries.

The thermal conductivity along and across the wood channels was measuredto be as low as 0.32 and 0.15 W m⁻¹ K⁻¹, respectively. When used as atransparent building material, the wood composite can provide improvedthermal insulation with respect to standard glass and in reducing airconditioning usage.

Furthermore, the subject transparent wood shows high impact absorptioncapability. When subjected to a sudden impact, the microchannels withinfiltrated polymer absorb and disperse the energy thus helping to keepthe wood from shattering. FIG. 10A illustrates the usage of transparentwood 26 as a sunlight harvesting rooftop. As shown in FIG. 10B, thetransmitted light intensity distribution is insensitive to the directionof the sun, keeping the indoor light consistent throughout the day. Theconductive heat flow can also be reduced with a more consistent indoortemperature. The transparent wood used as a window or rooftop materialwould pay for itself by providing cost savings in lighting and airconditioning energy indoors usage.

FIG. 11A is a scanning electron microscopy (SEM) image of a basswoodblock where the wood cells (channels) are naturally aligned along thedirection of growth.

There are many suitable choices for infiltration polymers as long as therefractive index is close to 1.5 and the material has a low viscosity.With the small refractive index mismatch between the cellulose and theepoxy, light can propagate along the growth direction while the woodcells (channels) function as lossy waveguides with a diameter rangingfrom tens to hundreds of micrometers depending on the species of naturalwood.

In order to show the light propagation in transparent wood 26, theDJ532-10 (Thorlabs, Inc.), 532 nm green single mode laser was used asthe incoming light source with a spot size of 200 μm. The beam isincident from the right hand side with a 45° input angle and isindicated by the arrow in FIG. 11A. A wood block with a large thicknessof 1.4 cm was used so that the propagation of the beam inside the woodblock can be clearly observed.

As can be seen in the top view of the wood composite, FIG. 11B, the beamquickly diverges after reaching the top surface of the wood and thenpropagates along the wood channel direction. The bright laser light iswell directed, indicating an efficient guiding effect. The lightconfinement inside the wood is mainly determined by the wood channelalignment direction instead of the incident light angle.

The densely packed and vertically aligned channels of the transparentwood 26 function as cylindrical broadband waveguides with highpropagation scattering losses. This unique light management capabilityof the transparent wood cells results in a macroscopic light propagationeffect with a large haziness. The optical properties including haze,forward transmittance, and backward reflection are summarized in FIG.11C. The results show that the transparent wood exhibits a hightransmittance around 90% and a simultaneously high optical haze around95%. By taking an averaging 90% transmittance and ≈10% reflection withinthe wavelength range from 500 to 1100 nm, a directional forward to backscattering ratio as high as 9 was obtained.

For comparison, nanostructures including nanocones and nanospheres areoften used in order to achieve directional scattering under the lightmanagement schemes using Mie scattering. However, the spectral responseis usually sensitive to wavelength and the forward to back scatteringratio is often less than that exhibited by transparent wood cells. Whileexhibiting a high transmittance, the haze of the transparent wood canexceed 95% which is likely due to the scattering of the verticallypropagating light by microstructural roughness.

The overall transparency for the transparent wood composite iscomparable to standard glass, plastic, and cellulose-based nanopaperconfirming the effectiveness of the herein developed procedure fortransparent wood composites.

FIG. 11D shows a schematic of the single mode laser at a tilted angleincident on a transparent wood sample 26 with the transmitted lightpattern captured on the screen. As shown in FIG. 11E, the beam intensitydoes not show notable deviation from a standard Gaussian distribution.Light management plays a crucial role in the effort to improve theoverall conversion efficiency of solar cells and light emitting diodes(LEDs). The subject transparent wood composites, with their unique lightmanagement capability, can serve as an effective transparent coating orsubstrate materials for building integrated photovoltaic.

FIG. 12A is representative of comparison studies of the haziness oftransparent wood, haze paper, and glass. In addition to the hightransmittance, the haze of the transparent wood composite reaches 95%and is much higher than that of the ultrahigh haze nanopaper, whichexhibits a typical haze value of ≈60%. In order to demonstrate theperformance of a transparent wood window as an efficient daylightharvesting building material with high haze and high transmittance, awooden house model (FIG. 12C) was built with a transparent wood roof 8cm×12 cm. Sources of glare include the morning and evening positions ofthe sun, ice, reflective surfaces on cars, highly polished floors, andthe windows of nearby buildings. Glare can interfere with the clarity ofa visual image. When used for daily applications, the transparent woodis shown to provide an effective antiglaring effect. When lookingthrough the transparent wood composite 26, glare is completely removedwhile a more uniform brightness is obtained as demonstrated in FIG. 12B.

In FIG. 12C, the effectiveness of using soda-lime glass and transparentwood, respectively, is compared as a light harvesting roof A solarsimulator from Oriel Instruments-Newport was used as the white lightsource. When incorporating a light harvesting building material into thehouse model, uniform indoor illumination was observed. In comparison toa glass rooftop, the high haze and high transparency of the woodcomposite resulted in maximized sunlight harvesting and a muchconsistent light distribution over the course of a day.

A calibrated Si detector from Thorlabs was used to evaluate the lightdistribution inside the house model. Six different spots were selectedand marked as 1-6 for the glass top house and the transparent wood tophouse, respectively. The results are shown in FIG. 12D. The maximumlight intensity inside the glass roof house is 12.3 mW cm⁻² while theminimum light intensity is only 0.35 mW cm⁻², making the illuminationnon-uniformity greater than 35. On the contrary, for the house with thetransparent wood rooftop, the light intensity difference betweenbrightest corner (4.9 mW cm⁻²) over the darkest corner (2.1 mW cm⁻²) is2.3. The transparent wood building material has been experimentallyshown to be an effective solution to save indoor lighting energy and toprovide uniform illumination with enhanced visual comfort and privacyprotection due to its intrinsic haziness.

In addition to the requirement for daylight harvesting and mechanicalstrength, transparent building materials must also meet the requirementsfor climate protection. Building materials for providing enhancedthermal insulation are therefore highly desirable. Effective insulationretards the flow of heat through the building shell and provides astructural barrier between the house and outside environment. If wellinsulated, the house stays warmer in the winter and cooler in the summer

The walls of most residential and commercial buildings are generallywell insulated with materials such as wood and composite foam. However,transparent building materials such as glass have a much higher thermalconductivity which results in higher heat flow than the surroundingmaterials and an overall reduction in thermal insulation of thebuilding. Thermal insulation from windows is particularly importantsince thermal bridging across transparent windows and roofs that aremade of glass can reduce energy efficiency and allow condensation.Current strategies to reduce heat loss through windows such as multiplelayer glazing are often costly and can add significant weight. On theother hand, wood is a natural insulator with air pockets in the cellstructure.

As shown in FIG. 13A, the transparent wood composites provide a highresistance to phonon traveling in the wood fiber microstructure. Theradial heat travelling pathway yields an even larger phonon scatteringeffect than that in the axial direction. The anisotropic thermalproperties of the transparent wood can be attributed to the alignment ofwood cells, which has been well-preserved after lignin removal andpolymer infiltration.

As can be seen in FIG. 13B, a thermal conductivity of around 0.32 W m⁻¹K⁻¹ was measured in the axial direction and 0.15 W m⁻¹ K⁻¹ in the radialdirection, comparable to the thermal conductivity of original basswood.A bulk polymer block (the same polymer that has been infiltrated intowood) shows a higher thermal conductivity of around 0.53 W m⁻¹K⁻¹. Theresulting lower thermal conductivity of transparent wood is due to thehigh phonon resistance across the wood cell walls (mainly cellulose andhemicellulose) and the multiple interfaces phonon scattering effect. Themechanical properties can further be tuned by the choice of theinfiltrating materials. In contrast, glass (Fisher ScientificMicroscopic Glass) has a much higher thermal conductivity measured to be≈1.0 W m⁻¹ K⁻¹ (FIG. 13B), showing that the transparent wood is moreeffective in reducing conductive heat flow.

In addition to their extreme light management capability, the mechanicalproperties of the transparent wood composites have been investigated.Glass has presented significant safety concerns when used as a buildingblock for residential and commercial structures. When glass undergoes asudden impact such as flying debris, an earthquake, or even suddenmovement of the occupants, glass can break and spray shattered pieces.Sometimes, glass can have sudden and spontaneous failure caused by edgeor surface damage which propagates through creep loads. The breaking ofglass requires immediate maintenance and attention, since the shatteredglass presents severe safety issues. On the other hand, wood canwithstand higher impact owning to the Van der Waals interactions betweenthe cellulose and the energy absorbing polymer infiltratedmicrostructure.

FIG. 14A shows the resulting morphology of glass and transparent wood 26after fracture due to a sudden hit from a dropping sharp object. Theglass shattered immediately into pointed pieces while theshock-resistant transparent wood stays intact. Glass is fairly rigid,but can be brittle as well. When glass is under a load it can onlyaccommodate stress to a relatively low level and then suddenly fail.Once a crack starts in glass, there is little within its structure tostop it from propagating. Consequently, glass exhibits a linear curve instrain and stress curve as shown in FIG. 14B. In contrast, the subjecttransparent wood composite possesses a much higher strain of 6%, morethan two orders higher than that of standard soda-lime glass. Thissubstantial increase in ductility is highly desirable for theapplication as a structural material. Even after breaking upon a suddenimpact, the transparent wood is only bent and split instead ofshattering into multiple sharp pieces. For commercial application as abuilding material, the transparent wood is also required to bewater-resistant.

The transparent wood sample 26 was immersed in water as shown in FIG.14C. After 72 hours, the sample was intact without any shape distortionor any degradation in mechanical and optical properties. The SEMobservation of the epoxy filled wood walls and the mechanical propertiesof the transparent wood after 72 hours of water immersion haveadditionally been studied. The results show that water has negligibleeffect on the properties of the transparent wood potentially due to theencapsulation of the polymer component.

Although this invention has been described in connection with specificforms and embodiments thereof, it will be appreciated that variousmodifications other than those discussed above may be resorted towithout departing from the spirit or scope of the invention as definedin the appended claims. For example, functionally equivalent elementsmay be substituted for those specifically shown and described, certainfeatures may be used independently of other features, and in certaincases, particular locations of the elements may be reversed orinterposed, all without departing from the spirit or scope of theinvention as defined in the appended claims.

What is being claimed is:
 1. Wood-based light transmitting system,comprising: a wood block pre-cut from a natural wood at a predeterminedangular relationship to a direction of natural internal channels in saidnatural wood and treated to remove lignin therefrom, thus forming alignin-devoid wood block, said natural internal channels having wallsformed of cellulose-containing material; and a filling material havingrefraction index substantially matching the refractive index of saidcellulose-containing material of said natural internal channels' walls,and substantially fully infiltrating said natural internal channels insaid lignin-devoid wood block with said filling material, therebyforming a transparent wood composite member.
 2. The wood-based lighttransmitting system of claim 1, wherein said transparent wood compositemember has an upper cut plane and a bottom cut plane, and wherein saidpredetermined angular relationship constitutes an angle of approximately90° between said direction of said natural internal channels and atleast one of said upper and bottom cut planes of said transparent woodcomposite member.
 3. The wood-based light transmitting system of claim1, wherein said transparent wood composite member has an upper cut planeand a bottom cut plane disposed in a direction substantially coincidingwith said direction of said natural internal channels in saidtransparent wood composite member.
 4. The wood-based light transmittingsystem of claim 1, wherein said filling material includes at least oneof materials selected from a group consisting of: Thermosettingpolymers, including Polyester fiberglass, Polyurethanes polymers,Vulcanized rubber, Bakelite, Duroplast, Urea-formaldehyde, Melamineresin, Diallyl-phthalate (DAP), Polyimides and Bismaleimides, Cyanateesters or polycyanurates, Furan resins, Polyester resins, Siliconeresins, Benzoxazine resins, Bis-Maleimides (BMI), Cyanate ester resins,Epoxy (Epoxide) resins, Phenolic (PF) resins, Polyester resins,Polyimides, Polyurethane (PUR) resins, Silicone Resins, Vinyl esterresins, Thermoplastic polymers, including Acrylic, ABS, Nylon, PLA,Polybenzimidazole, Polycarbonate, Polyether sulfone, Polyetheretherketone, Polyetherimide, Polyethylene, Polyphenylene oxide, Polyphenylenesulfide, Polypropylene, Polystyrene, Polyvinyl chloride, Teflon, andCellulose derivatives, including, Cellulose acetate, Cellulose acetatebutyrate, Cellulose triacetate, Methyl cellulose, Hydroxypropyl methylcellulose, Ethyl cellulose, Hydroxyethyl cellulose, Carboxymethylcellulose, Dissolved cellulose, Nanofibrillated cellulose, Cellulosenanocrystals, functional index matching materials, such as, for example,liquid crystal, pressure/temperature sensing materials, piezoelectricmaterials, colorless polymer nano-glue, transparent liquid epoxy resinprecursor with low viscosity, a mixture of a resin and non-blushingcycloaliphatic hardener, polyvinylpyrrolidone (PVP), Poly(methylmethacrylate) (PMMA), Poly(vinyl alcohol) (PVA), andPolydimethylsiloxane (PDMS).
 5. The wood-based light transmitting systemof claim 1, wherein said transparent wood composite member is configuredas a block having a length and width, respectively, of approximately 1mm and larger, and a thickness of approximately 100 μm and larger. 6.The wood-based light transmitting system of claim 1, wherein saidtransparent wood composite member has light transmittance rangingapproximately from 80% to 95% and optical haze ranging approximatelyfrom 80% to 100% in the visible light wavelength range from 400 nm to1100 nm.
 7. The wood-based light transmitting system of claim 1, whereinthe refractive index of said filling polymer is approximately 1.53 at alight wavelength λ=550 nm.
 8. The wood-based light transmitting systemof claim 1, wherein said wood-based light transmitting system furtherincludes an optoelectronic system including at least one of photonicsystems, solar cells, photo-detectors, displays, and wide-angle lightingsystems having advanced light management.
 9. The wood-based lighttransmitting system of claim 8, wherein said at least one of the solarcells includes an optically active layer and said at least onetransparent wood composite member shaped as a layer having a thicknessranging between 100 μm and 3 mm, and disposed in optical contact withsaid optically active layer.
 10. The wood-based light transmittingsystem of claim 1, wherein said transparent wood composite memberexhibits high mechanical strength with a fracture strength of 23.5-45MPa and higher and high ductility, and wherein said wood-based lighttransmitting system includes light-harvesting building structures.
 11. Atransparent wood composite, comprising: a wood block of predetermineddimensions pre-cut from a natural wood in a predetermined angularrelationship to a direction of natural internal channels of said naturalwood and treated to form a lignin-devoid wood block, said naturalinternal channels in said wood block having walls formed of naturalcellulose-containing material, and a filling polymer substantiallycompletely infiltrating said internal channels in said lignin-devoidwood block and cross-linked with said cellulose-containing material ofsaid internal channels' walls in said wood block, wherein said fillingpolymer has refractive index substantially matching the refractive indexof said natural cellulose-containing material of said internal channels'walls.
 12. The wood-based light transmitting system of claim 11, whereinsaid wood block is configured with an upper cut plane and a bottom cutplane spaced apart each from another by a pre-determined distanceranging approximately between 100 μm and 14 mm, and wherein at least oneof said upper and bottom planes extends in crossing relationship withsaid natural internal channels or substantially therealong.
 13. Thewood-based light transmitting system of claim 11, wherein said fillingpolymer has refractive index approximating to 1.48.
 14. The wood-basedlight transmitting system of claim 13, wherein the refractive index ofsaid filling polymer is approximately 1.53 at the light wavelength ofλ=550 nm.
 15. The wood-based light transmitting system of claim 11,wherein said filling polymer includes at least one of the polymersselected from a group consisting of: Thermosetting polymers, includingPolyester fiberglass, Polyurethanes polymers, Vulcanized rubber,Bakelite, Duroplast, Urea-formaldehyde, Melamine resin,Diallyl-phthalate (DAP), Polyimides and Bismaleimides, Cyanate esters orpolycyanurates, Furan resins, Polyester resins, Silicone resins,Benzoxazine resins, Bis-Maleimides (BMI), Cyanate ester resins, Epoxy(Epoxide) resins, Phenolic (PF) resins, Polyester resins, Polyimides,Polyurethane (PUR) resins, Silicone Resins, Vinyl ester resins,Thermoplastic polymers, including Acrylic, ABS, Nylon, PLA,Polybenzimidazole, Polycarbonate, Polyether sulfone, Polyetheretherketone, Polyetherimide, Polyethylene, Polyphenylene oxide, Polyphenylenesulfide, Polypropylene, Polystyrene, Polyvinyl chloride, Teflon, andCellulose derivatives, including, Cellulose acetate, Cellulose acetatebutyrate, Cellulose triacetate, Methyl cellulose, Hydroxypropyl methylcellulose, Ethyl cellulose, Hydroxyethyl cellulose, Carboxymethylcellulose, Dissolved cellulose, Nanofibrillated cellulose, Cellulosenanocrystals, functional index matching materials, such as, for example,liquid crystal, pressure/temperature sensing materials, piezoelectricmaterials, colorless polymer nano-glue, transparent liquid epoxy resinprecursor with low viscosity, a mixture of a resin and non-blushingcycloaliphatic hardener, polyvinylpyrrolidone (PVP), Poly(methylmethacrylate) (PMMA), Poly(vinyl alcohol) (PVA), andPolydimethylsiloxane (PDMS).
 16. A method of fabrication of a wood-basedlight transmitting system, comprising: (a) pre-cutting a wood block froma natural wood in a pre-determined angular relationship to naturalinternal channels of the natural wood, the natural internal channelshaving walls formed from cellulose-containing material and being filledwith lignin, (b) removing the lignin from natural internal channels ofsaid wood block, thus forming a lignin-devoid wood block, and (c)sequentially infiltrating said natural internal channels in saidlignin-devoid wood block with a filling polymer having refractive indexsubstantially matching a refractive index of said cellulose-containingmaterial of the internal channels' walls.
 17. The method of claim 16,further comprising: in said step (b), preparing a lignin removalsolution by mixing a solution of NaOH in deionized water, and a solutionof Na₂SO₃ in deionized water, boiling said pre-cut wood block in saidlignin removal solution for approximately 12 hours, rinsing said pre-cutwood block in hot distilled water, and boiling said rinsed pre-cut woodblock in a bleaching solution containing solution of H₂O₂ in distilledwater until a color of the pre-cut wood block disappears, therebyobtaining a lignin-devoid wood block.
 18. The method of claim 16,further comprising: in said step (c), immersing said lignin-devoid woodblock in said filing polymer in the liquid phase thereof, degassing saidliquid filing polymer under pressure of 200 Pa for approximately 5-10minutes to remove a gas and ethanol solvent from the lignin-devoid woodblock, applying the atmosphere pressure to the liquid filling polymer topromote the internal channels infiltration process, repeating saidatmosphere pressure application a predetermined number of times, thusobtaining the polymer infiltrated wood block immersed in the liquidfilling polymer, maintaining the polymer infiltrated wood block in saidfilling polymer undisturbed at approximately 30° C.-60° C. forapproximately 12 hours until the liquid filling polymer solidifies, andremoving said polymer infiltrated wood block from the solidified fillingpolymer, thus obtaining said transparent wood composite member.
 19. Themethod of claim 16, further comprising: fabricating a solar cellincluding an optically active layer, depositing ethanol on a surface ofsaid optically active layer, placing said transparent wood compositemember on said optically active layer in contagious contact therewith,thus forming a sandwich structure, and drying the sandwich structurecontaining said transparent wood composite member coupled to saidoptically active larger at room temperature to firmly attach saidtransparent wood composite member to the solar cell.
 20. The method ofclaim 16, further comprising: attaching said transparent wood compositemember to a building at a site of at least one of a window and a roof.